The cellular integrity and response to hypoosmotic conditions in the yeast Saccharomyces cerevisiae are ensured by a MAP kinase signal transduction pathway mediated by the yeast homolog of mammalian protein kinase C. Bck1p functions as the MAP kinase kinase kinase of this pathway. Here we report on the cloning and analysis of the BCK1 homolog from the milk yeast Kluyveromyces lactis (KlBCK1). The deduced protein sequences display three highly conserved domains with the serine/threonine kinase domain containing 89 % identical amino acid residues. Interestingly, a region identified in KlBck1p as a putative SAM domain, mediating protein-protein interactions, is also conserved in ScBck1p. Yet, two-hybrid analyses indicate that this region may not be involved in dimerization of KlBck1p in contrast to its S. cerevisiae counterpart. Expression of KlBCK1 fully complements the defects in a Scbck1 null mutant and is capable of activating the pathway as indicated by a reporter system based on the transcription factor Rlm1p. However, deletion from the haploid K. lactis genome does not result in a loss of cellular integrity under a variety of conditions tested. Thus, despite the functional conservation in this component of the MAP kinase pathway in both yeast, cellular integrity in K. lactis may depend at least in part on different signalling mechanisms when compared with S. cerevisiae.